I recently took the decision to share some of my older blog posts in order to pad out this iteration of the blog a bit and get me back into the swing of things. It beats ranting about Jurassic World and provides some hopefully readable content. Here is a piece I wrote in August 2010 as a bit of pop. science writing.
With the vast size of the Earth’s oceans, it is not unlikely that many of us will swim in them from time to time. Whether you are going for an innocent paddle, catching waves on your surfboard, or sailing the seas on a fishing trawler or pirate ship, you will be sharing the waters with myriad different animals, some of which are dangerous. If you were to find yourself swimming under water with a big, moving animal coming ever closer, how do you know whether you have a friendly dolphin swimming towards you or whether it might be a hungry shark? Well, the easiest way can be done at a distance and is a simple observation with an interesting explanation – evolution.
Sharks, on the other hand, have a vertical tail fluke and so must flex their body from side to side for propulsion. If you see the vertical tail fluke or side to side motion, then what is coming towards you is a fish and so might be a shark.
As our fishy ancestors used a side by side motion to propel themselves through water, so did our earliest terrestrial ancestors and so do reptiles today. Snakes are an extreme example of this sort of movement, but the side to side motion is still there. During the Mesozoic era things began to change, as our ancestors (and convergently in dinosaurs too) developed a more upright posture, instead of the sprawling gait of reptiles. With an erect posture the more effective way to rapidly move is to flex the spine up and down whilst running, rather than side by side.
Many vertebrates have some of their vertebrae fused to facilitate particular movements, so future evolution can often be restricted to working within the confines of that movement. As dolphins evolved from terrestrial mammals, their semi-aquatic ancestors also used this up and down movement and so adapted this to movement in the water. Side to side motion, like that of a shark, would require a larger number of changes when there was the simpler solution of up and down movement (though note that evolution does not have the foresight, it simply uses what is available – quick fix solutions often work in evolution). The motion of whales and dolphins is testament to their ancestry, having descended from active land mammals.